1. Field of the Invention
This relates to current limiting drivers for driving loads that require a turn-on current that is higher than the desired maintenance current magnitude.
2. Prior Art
Various overload protection circuits using current limiting are known. For example, U.S. Pat. No. 3,801,894 issued to Spiegel teaches a power supply in which the output of a comparator amplifier is applied to a current limiting transistor, which in turn limits the load current upon the occurrence of a short circuit in the load. A feedback means is provided such that the load voltage is compared to a reference voltage. When the load voltage exceeds the reference voltage, the comparator turns on, thereby also turning on the current limiting transistor. The current limiting transistor serves as a bias control such that, when turned on, it limits the conductivity of output control transistors, thus limiting the output current.
U.S. Pat. No. 3,470,457 issued to Howlett teaches a voltage regulator which utilizes a cascaded amplifier pair to provide constant voltage at its output. The first amplifier serves as a voltage stabilizer, producing constant output voltage to the second amplifier. The second amplifier acts as an inverter and gain device. Feedback is provided from the second amplifier to the positive input of the first amplifier to stabilize the voltage of the first amplifier. However, the Howlett circuit does not produce a latching operation to stabilize the voltage of the second transistor.
U.S. Pat. No. 3,753,079 issued to Trilling teaches a foldback current limiter in which a diode is used as the regulating device. A conventional current limiter is provided with a Darlington pair power amplifier and a diode connected in shunt to the base and the emitter of a transistor within the current limiter. Should a short circuit occur in the load, the voltage across the diode is increased such that the base emitter voltage of the transistor within the current limits your increases to a point where the transistor is turned on. Since the collector of this transistor is connected to the emitter of the main current limiter transistor, this also serves to reduce the conduction of the main current limiter transistor, thereby serving ultimately to limit the load current. Subsequent operation of the current limiter is governed by the first transistor. That is, subsequent to initiating the current limiting operation, the input transistor controls the limiting operation. It would be advantageous to have the limiting operation carried out independently of the subsequent voltage of the input transistor.
U.S. Pat. No. 4,051,392 issued to Rosenthal et al discloses a startup current limiter driver in which the load current increases to a certain point, and is constant thereafter. A starting circuit initiates current flow and is automatically disconnected from the circuit limiter once the limiter is fully energized. A first transistor is provided by the starter circuit which sequentially energizes the transistors of the current limiter. When the transistors within the current limiter are sufficiently energized, such that the voltage at a common node reaches a particular value, a second transistor is turned on in the starting circuit to turn off the first transistor. However, it would be desirable that the internal voltage transition point is not a function of turn-on voltages of transistors within the current limiter but only a function of preselected resistor values and configurations. Further, the above cited patents are intended as overload protection circuits and return to normal operation when the overload condition is removed. It would be desirable to use a circuit in which normal operation provides an initial application of full voltage to the load until a predetermined current is reached, at which time the current folds back to a specific current limit level and latches at that current level until the input to the circuit is removed. These are some of the problems this invention overcomes.